1. Field of the Invention
The present invention relates to a liquid crystal display element including high molecular weight ferroelectric liquid crystals (referred to as “high molecular FLCs”) as a liquid crystal material, as well as a method of manufacturing the same.
2. Discussion of the Related Art
Recently, as an image display device with improved features such as a small thickness, light weight and reduced energy consumption, a liquid crystal display (‘LCD’) device is widely used. The majority of LCD devices conventionally known in the art employ nematic liquid crystals (‘NLCs’).
As shown in FIG. 8, an LCD element containing NLCs generally consists of two opposing substrates 51, transparent electrodes 52 respectively provided at opposing sides of the facing substrates 51, alignment films 53 which are oriented by a rubbing method and provided on each of the transparent electrodes 52 of the substrates 51, and a liquid crystal layer 54 formed by introducing (or charging) nematic liquid crystals between the substrates 51.
Various LCD modes using NLCs such as TN (Twisted Nematic), ECB (Electrically Controlled Birefringence), STN (Super Twisted Nematic), IPS (In-Plane Switching) and VA (Vertical Alignment), etc. are currently employed. However, NCL-based LCD elements do not have any inherent bistability (memory property), although they do enable continuous tone display.
The NLC-based LCD element shows improved uniformity in orientation, thus enabling displays with a high contrast ratio. In order to orient NLCs, an alignment film oriented by a rubbing method may be used, that is, a rubbing method may be employed in orientation of NLCs. Although an NLC-based LCD element can achieve response times suitable for standard household televisions and the like (suitable for standard motion picture playback), inherent properties of NLCs make high-speed response of less than 1 ms difficult to achieve.
Therefore, in order to improve the response time of an LCD element, a surface stabilized-ferroelectric liquid crystal (SS-FLC) mode LCD element formed using low molecular weight ferroelectric liquid crystals, instead of NLCs, has been proposed. Such SS-FLC mode LCD elements employing low molecular weight ferroelectric liquid crystals (referred to as “low molecular FLCs”) has a structure in which low molecular FLCs are applied to a liquid crystal layer 54 shown in FIG. 8.
The SS-FLC mode LCD element with low molecular FLCs may improve response time compared to the NLC-based LCD elements. However, due to inherent bistability of the SS-FLC mode LCD element, this LCD element cannot easily embody continuous tone display. Continuous tone display provided by the SS-FLC mode LCD element employing low molecular FLCs entails specific technological challenges, for example, area coverage gradation, domain gradation, frame gradation, etc. (see Japanese Unexamined Patent Publication No. S62-131225), complex structures, high production costs, and so forth.
In SS-FLC mode LCD elements employing low molecular FLCs, the low molecular FLCs have a layered structure and show reduced orientation stability, compared to an NLC-based LCD element. Other problems such as difficulty in uniform orientation, reduced contrast ratio as compare to NLC-based LCD elements, and the like are encountered. Moreover, in order to orient NLCs, an alignment film oriented by a rubbing method may be used, that is, a rubbing method may be employed in orientation of NLCs.
At the expense of inferior bistability of the SS-FLC mode LCD element containing low molecular FLCs, Half-V (H-V) mode or V mode LCD elements to provide continuous tone display have been disclosed (see Japanese Unexamined Patent Publication No. 2004-86116).
The H-V mode or V mode LCD element employing low molecular FLCs has been proposed as a fast response time version of NCL-based LCD elements.
More particular, such H-V mode or V mode LCD element employing low molecular FLCs provides faster response times than NLC-based LCD elements and, in addition, may enable continuous tone display at the expense of reduced bistability.
For the H-V mode or V mode LCD element employing low molecular FLCs, the low molecular FLCs have a layered structure and show worse orientation stability than NLC-based LCD elements. Other problems such as difficulty in uniform orientation, decreased contrast ratio as compared to NLC-based LCD elements, and the like, are encountered. Moreover, in order orient NLCs, an alignment film oriented by a rubbing method may be used, that is, a rubbing method may be employed in orientation of NLCs.
In order to enhance orientation stability of the SS-FLC mode LCD element containing low molecular FLCs, improved SS-FLC mode LCD elements containing high molecular FLCs have been disclosed (for example, Japanese Unexamined Patent Publication Nos. S56-107216; H02-240192; H02-271326; H03-42622; and H06-281966).
As shown in FIG. 9, such an SS-FLC mode LCD element employing high molecular FLCs generally includes two opposing substrates 61, a transparent electrodes 62 respectively provided on facing sides of the substrates 61, and a liquid crystal layer 63 formed by introducing high molecular FLCs between the substrates 61. In this regard, provision of shear stress to the substrates 61 (a shearing method) while applying voltage between two substrates 61 may achieve orientation of the high molecular FLCs, thereby achieving orientation of the liquid crystal layer 63.
The SS-FLC mode LCD element employing high molecular FLCs exhibits a response time comparable to that of an NLC-based LCD element, although shortcomings such as a slower response (that is, a longer response time) than that of a low molecular FLC-based SS-FLC mode LCD element, due to the high molecular weight and relatively high viscosity of the FLCs, are encountered. Since the SS-FLC mode LCD element employing high molecular FLCs exhibits inherent bistability, simple continuous tone display is not possible. In order to perform continuous tone display, specific technologies such as area coverage gradation, domain gradation, frame gradation, etc. as described above are further required. However, in such case, some problems such as the necessity of complex structures, high production costs, etc. are encountered.
Since the SS-FLC mode LCD element employing high molecular FLCs has a layered structure of high molecular FLCs as well as a relatively high molecular weight, orientation stability is superior to that of an LCD element containing low molecular FLCs, while being worse than that of NLC-based LCD elements. Owing to such a layered structure comprising high molecular FLCs, it is difficult to provide uniform orientation. Therefore, the contrast ratio of the foregoing LCD element is not comparable to that of the NLC-based LCD element. Further, since their high molecular weight means that uniform orientation is not readily obtained using a rubbing method, a more complex shearing method should be used to provide uniform orientation.
In addition, conventional NLC-based LCD elements, low molecular FLC-based LCD elements (SS-FLC mode, H-V mode, V mode) and high molecular FLC-based LCD elements (SS-FLC (shearing method)) all enable continuous tone display, while not having continuous gradation memory properties to maintain (or memory) the current state of gradation when voltage is interrupted. Here, FIG. 10 shows a list of different display properties of conventional LCD elements.
A relation (a potential curve) between orientation angles (varied by voltage application) of an LCD element to enable continuous tone display (for example, NLCs, H-V mode, V mode) and potentials, is shown in FIG. 11. As can be seen from FIG. 11, use of an alignment film oriented using a rubbing method has a significant influence upon the potential curve.
For the SS-FLC mode LCD element having bistability with lower molecular or high molecular FLCs, liquid crystal molecules move to a bistable position upon voltage application. A relation (a potential curve) between orientation angles of the SS-FLC mode LCD element containing low molecular or high molecular FLCs and potentials is shown in FIG. 12.
As described above, conventional LCD elements cannot retain a set gradation once voltage is no longer applied thereto.
Therefore, the conventional LCD element entails a problem in that it does not exhibit a gradation memory property to maintain (or memory) the current gradation at continuous tone display when voltage is interrupted, while still performing the continuous tone display.
Under such circumstances, application of a domain gradation to provide continuous gradation memory properties has been disclosed (for example, see Hideo Fujikake et al. “Polymer-Stabilized Ferroelectric Liquid Crystal Devices with grayscale Memory”, Jpn. J. Appl. Phys, Vol. 36, pp. 6449-6454, 1997).